Radio frequency identification (RFID) tags are now widely used to mark, inventory and track various products. RFID tags generally transmit to a handheld or robotically controlled reader device a radio frequency signal that includes product information. RFID tags generally include an integrated circuit for storing and processing information, a transceiver for transmitting and receiving RF signals, and an antenna. Some RFID tags are active RFID tags and include their own battery power source. Passive RFID tags do not have their own power source and require receiving a power signal from a reader device to operate. For interrogating passive RFID tags, a reader generally transmits a continuous wave (CW) or modulated radio frequency (RF) signal to a tag. The tag receives the signal, and responds by modulating the signal and then “backscattering” an information response signal to the reader. The reader receives the response signal from the tag, and the response signal is demodulated, decoded and further processed.
Although RFID tags are often read individually or serially, many applications require large groups of RFID tags to be read simultaneously or near simultaneously. For example, crates or pallets including large numbers of products may need to be interrogated using a single RFID reader device. Just one signal from the reader device therefore may be used to activate numerous RFID tags and request information from the tags. Further, such crates or pallets may be adjacent other similar crates or pallets, and activating RFID tags in such adjacent crates or pallets may not be desired. Therefore, use of directional antennas in reader devices and other means for localizing an RFID interrogation signal have been used.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.